The circuit design of electronic components used in TVs, radios, computers, medical devices, office machinery and communications apparatus is becoming increasingly complex. For example, with the manufacture of these devices and integrated circuits equivalent to hundreds of thousands of transistors for other devices, designs are becoming more complex, while at the same time electronic components are becoming smaller, even larger numbers of components are being built into ever smaller surface areas, and devices are continually becoming more compact.
These electronic components, in particular IC packages such as CPUs mounted on printed circuit boards, suffer decreased performance due to the temperature rise accompanying the heat emitted during use, and this leads to defects and loss of function. To solve this problem, in the related art, a heat dissipating sheet or heat dissipating grease with good heat conduction properties was interposed between the IC package and a heat-radiating fin. However, as components continue to become more compact and their performance improves, their heat emission amount is increasing year by year, so a heat-dissipating member having excellent heat-dissipating properties was desired.
In particular, in recent years, the CPUs used in electronic instrument such as personal computers, digital video disks and portable telephones, or the LSIs in driver IC or memories, are becoming more highly integrated and their operation is becoming faster, so their power consumption is increasing. This increased heat emission is one reason for faulty operation of electronic equipment or damage to electronic components, so an efficient way of dealing with heat emission is an important issue.
In the related art, in electronic equipment, a heat sink using a metal plate having a high thermal conductivity such as yellow copper or the like, was used to suppress the temperature rise of electronic components during use. This heat sink conducts heat produced by the electronic components, and discharges it to the surface due to the temperature difference with the outside air.
In order for a heat sink to efficiently conduct heat produced by an electronic component, the heat sink must be in intimate contact with the electronic component. However, there is a difference in height of various components and a tolerance in assembly procedures, so a flexible thermally-conducting sheet or thermally-conducting grease was interposed between the electronic component and the heat sink, and heat from the electronic component was conducted to the heat sink via this thermally conducting sheet or thermally conducting grease.
The heat-dissipating sheet of the related art has the advantage it can be easily mounted, but from the viewpoint of workability in the manufacturing process, there is a limit to the thermally-conducting filler it can contain, and as there was a high boundary thermal resistance when it was installed, it did not exhibit its full heat-dissipating properties in practice.
The aforesaid thermally-conducting sheet was a thermally-conducting sheet formed, for example, from a thermally-conducting silicone rubber (thermally-conducting silicone rubber sheet) and the thermally-conducting grease was a thermally-conducting silicone grease. However, with the thermally-conducting silicone rubber sheet used in the related art, gaps are produced in the interface with the electronic component, so boundary contact resistance increased and thermally-conducting properties were inadequate. This defect is a major problem for cooling a high frequency-driven CPU which has a large heat emission, so the reduction of boundary contact resistance was strongly desired.
Termally-conducting grease on the other hand, is close to a liquid, and its boundary contact resistance can practically be ignored, in addition to which it has good thermally-conducting properties. However, a special dispenser must be installed, and it is not easy to recover it. Further, if the thermally-conducting grease is subjected to heat cycles between room temperature and the operating temperature of the electronic component (60-120° C.) for long periods of time, a problem of pump-out arises. In pump-out, liquid oil ingredients contained in the grease separate and ooze out from between the electronic component and heat-dissipating member, so the grease solidifies, and cracks or voids appear in it. As a result, the thermal resistance increases, and the heat from the electronic component can no longer be dissipated. Therefore, although heat-dissipating grease has the advantages that it can follow and be in contact with coated surfaces without being affected by imperfections in the surface of the CPU or heat-dissipating fin, and its boundary thermal resistance is low, it soils other components, and the oil in it oozes out after long periods of use.
To resolve the above problems, a phase-change heat-dissipating member (phase change sheet), which is a solid sheet at room temperature, but which softens due to the heat released during operation of electronic components so that its boundary contact resistance reaches a negligible level, has already been proposed. For example, U.S. Pat. No. 4,466,483 discloses a phase-changing wax layer formed on both surfaces of a non-metal sheet, and U.S. Pat. No. 5,904,796 discloses a phase-changing paraffin or petroleum jelly formed on one surface of a metal foil and a layer formed on the other surface. Further, JP-A No. 2000-509209 (Koho) discloses a phase change sheet comprising an acrylic binder, wax and thermally-conducting filler, wherein an interlayer comprising a reticular structure or film is not provided.
However, in the aforesaid related art, from the viewpoint of ease of processing and operation, there was a limit to the thermally-conducting filler material amount. Also, even after a phase transition, the contact surfaces are in intimate contact with just resin ingredients as the main body. Therefore, although increase of boundary contact resistance could be prevented, as the heat conductivity of the resin itself is low, the demand for further reduction of boundary contact resistance could not be satisfied.
The Inventor carried out intensive studies to solve the above problems. A heat-dissipating member having excellent heat-dissipating properties was thereby discovered which is a solid at ordinary temperature, can assume any required shape including that of a sheet, and by allowing a complete phase transition of the thermally-conducting filler in the uncured ingredients, permits a remarkable reduction of boundary contact resistance. It was also discovered that when this composition is manufactured, it has excellent homogeneity, and can easily be molded into any desired shape including that of a sheet by using a low melting point metal powder of controlled particle diameter, and blending/kneading this under temperature conditions below its melting point.
Specifically, a composition obtained by selecting a silicone resin which is a solid at ordinary temperature, which softens within a fixed temperature range and which has a low viscosity or melts, and if necessary uses a thermally-conducting powder having a melting point of 250° C. or higher as filler, is disposed between a heat dissipating electronic component and heat-dissipating component (boundary), and attains the desired heat dissipation by causing a phase transition in the low melting point metal rather than the resin.
In particular, in the case of a heat-dissipating member which is a solid sheet at ordinary temperature containing a low melting point metal which can easily be attached to or removed from an electronic component or heat sink, the boundary contact resistance was reduced to a negligible level, and excellent heat-dissipating performance was obtained over long periods of time without pump-out. This is due to melt softening of the low melting point metal rather than the resin by the heat emitted during operation of the electronic component, or by temporarily applying heat above the melting point of the low melting point metal contained in the composition during installation, and this led to the present invention.
It is therefore a first object of this invention to provide a heat-dissipating member having heat-dissipating properties superior to those of the heat-softening heat-dissipating members of the related art, which uses a phase transition of a low melting point metal.
It is a second object of this invention to provide a heat-dissipating member which is a sheet or film at ordinary temperature, which has a sufficiently small boundary contact resistance in use, and which has excellent heat dissipation properties over long periods of time without pump-out.
It is a third object of this invention to provide a method of manufacturing a heat-dissipating member having heat-dissipating properties superior to those of the heat-softening heat dissipating members of the related art, using a phase transition of a low melting point metal.
It is a fourth object of this invention to provide a method of manufacturing a heat-dissipating member which is a sheet or film at ordinary temperature, and which is reversibly melt softened by the action of heat in use.
It is a fifth object of this invention to provide an installation method which makes full use of the performance of the heat-dissipating member of this invention.